Thermal cracking of hydrocarbon feeds in the presence of steam (“steam cracking”) is a commercially important technology for producing light olefins such as ethylene, propylene, and butadiene. Typical hydrocarbon feeds include, e.g., one or more of ethane and propane, naphtha, heavy gas oils, crude oil, etc. Steam cracking furnaces for carrying out steam cracking generally include a convection section, a radiant section located downstream of the convection section, and a quenching stage located downstream of the radiant section. Typically, at least one burner is included in the steam cracking furnace for providing heat to the convection and radiant sections. The burners are typically located in at least one firebox, the firebox being proximate to the radiant section, with the convection section being located downstream of the radiant section with respect to the flow of heated gases (typically combustion gases) produced by the burner. Tubular conduits (‘tubes”) are utilized for at least (i) conveying the hydrocarbon feed, steam, and mixtures thereof through the furnace's convection and radiant sections, (ii) transferring heat to the hydrocarbon and/or steam inside the tube for the steam cracking reactions, (iii) conveying product effluent away from the radiant section, (iv) conveying a decoking mixture for removing coke, e.g., from inside convection tubes and/or radiant tubes, and (v) conveying decoking effluent away from the radiant section. Certain steam cracker tubes are heat-transfer tubes. Typically, heat-transfer tubes located in the convection section are called “convection tubes”, and those located in the radiant section are called “radiant tubes”. When the convection tubes and/or radiant tubes are arranged in coils, it is typical to call these “convection coils” and “radiant coils”.
In one conventional process, a hydrocarbon feed is introduced into at least one of the convection coils. The convection coil's external surface is exposed to the heated gases conducted away from the burner. The hydrocarbon feed is preheated by indirectly transferring heat from the heated gases to hydrocarbon feed located inside the convection coil. Steam is combined with the pre-heated hydrocarbon feed to produce a hydrocarbon+steam mixture. At least one additional convection coil is utilized for pre-heating the hydrocarbon+steam mixture, e.g., to a temperature at or just below the temperature at which significant thermal cracking occurs.
The preheated hydrocarbon+steam mixture is conducted via cross-over piping from the convection section to at least one radiant tube located in the radiant section. Conventional radiant tubes are typically formed from a steam cracker alloy comprising chromium, iron, and nickel, as well as various other elements, usually in low concentration, e.g., ≤5.0 wt. %, to obtain desired performance. The preheated hydrocarbon+steam mixture is indirectly heated in the radiant tube, primarily by the transfer of heat from the burner to the radiant tube's exterior surface, e.g., radiant heat transfer from flames and high temperature flue gas produced in one or more burners located in the fire box, radiant heat transfer from the interior surfaces of the firebox enclosure, convective heat transfer from combustion gases traversing the radiant section, etc. The transferred heat rapidly raises the temperature of the pre-heated hydrocarbon+steam mixture to the desired coil outlet temperature (COT), which typically ranges from 1450° F. (788° C.) for some very heavy gas oil feeds to 1650° F. (871° C.) for ethane or propane feeds.
Heat transferred to the preheated hydrocarbon+steam mixture located in one or more of the radiant tubes results in thermal cracking of at least a portion of the mixture's hydrocarbon to produce a radiant coil effluent comprising molecular hydrogen, light olefin, other hydrocarbon byproducts, unreacted steam, and unreacted hydrocarbon feed. Transfer line piping is typically utilized for conveying radiant coil effluent from the radiant section to the quenching stage. Coke accumulates in the furnace during the thermal cracking, e.g., on internal surfaces of the convection tubes and especially on internal surfaces of the radiant tubes. After an undesirable amount of coke has accumulated, a flow of decoking mixture, typically an air-steam mixture, is substituted for the hydrocarbon+steam mixture for removing accumulated coke. Decoking effluent is conducted away. Following coke removal, the flow of hydrocarbon+steam mixture is restored to the decoked tubes. The process continues, with alternating pyrolysis (thermal cracking) mode and decking mode.
Selectivity to light olefins during pyrolysis mode is favored by short contact time, high temperatures, and low hydrocarbon partial pressures. For this reason radiant tubes typically operate at a temperature (measured at the tube metal) as high as 2050° F. (1121° C.). Radiant tubes are therefore manufactured from alloys having desirable properties at high temperature, such as high creep-strength and high rupture-strength. Since the tubes are exposed to a carburizing environment during hydrocarbon pyrolysis, the alloy is typically carburization-resistant. And since the tubes are exposed to an oxidizing environment during decoking, the alloy is typically oxidation-resistant. Conventional heat-transfer tube alloys include austenitic Fe—Cr—Ni heat resistant steels having variations of steam cracker alloys based on a composition having 25 wt. % chromium and 35 wt. % nickel (referred to as a “25 Cr/35 Ni alloy”), or a composition having 35 wt. % chromium and 45 wt. % nickel (referred to as a “35 Cr/45 Ni alloy”). It is conventional to employ differing compositions of minor alloying elements in order to enhance high temperature strength and/or carburization resistance.
In conventional alloys, a surface oxide comprising Cr2O3 typically forms during pyrolysis. This oxide is believed to protect iron and nickel sites from contact with the hydrocarbon during pyrolysis mode, thereby lessening the amount of undesirable coke formation. It is observed, however, that under more severe pyrolysis conditions, e.g., conditions typically utilized for increasing light olefin yield, the formation of this protective oxide layer is suppressed in favor of carbon-containing phases, e.g., Cr3C2, Cr7C3, and/or Cr23C6. Accordingly, discontinuities develop over time in the carburization-resistant scale located on the tube's inner surface, resulting in iron and nickel exposure to the hydrocarbon feed, leading to an increase in the rate of coke formation.
In an attempt to overcome this difficulty, U.S. Patent Application Pub. No. 2012/0097289 discloses increasing the tube's carburization resistance by employing an alloy containing 5 to 10 wt. % aluminum. The alloy is said to form an Al2O3 scale during pyrolysis mode. It is reported that an Al2O3 scale remains in a stable oxide even under conditions where chromium preferentially forms carbides rather than oxides. Since such carburization-resistant alloys have a lower creep-strength and lower rupture-strength than do conventional heat-transfer tube alloys that do not contain aluminum, the reference discloses a tube structure wherein a continuous inner member formed from the aluminum-containing alloy is bonded to the inner surface of a tubular outer member which comprises a higher-strength alloy. While such tubes suppress coke formation, their dual-layer construction is economically demanding.
It is conventional to lessen the amount of aluminum in the steam cracker alloy in order to increase strength and thereby obviate the need for an outer member. See, e.g., U.S. Pat. No. 8,431,230, which discloses an aluminum-containing steam cracker alloy comprising 2 to 4 wt. % aluminum.
It is also conventional to increase the tube's heat transfer efficiency in order to expose the hydrocarbon+steam mixture to higher temperature and shorter contact time during pyrolysis, resulting in better selectivity for light olefin production. For example, increasing the heat transfer by increasing the tube's surface area that is exposed to the hydrocarbon feed is described in U.S. Pat. Nos. 6,419,885 and 6,719,953. Other methods for increasing the tube's heat transfer efficiency include the application of a mixing element (sometimes referred to as a “bead” or “fin”) on the inner surface of the heat transfer tube. For example, U.S. Pat. No. 5,950,718 describes the use of a conventional 25 Cr/35Ni tube that includes a helical mixing element that is applied to the tube inner surface by plasma powder welding or arc welding. It has been observed that the flow of hydrocarbon+steam mixture through a radiant tube during pyrolysis results in the formation of a boundary layer adjacent to the radiant tube's inner surface. The boundary layer comprises hydrocarbon. The mixing element disturbs the boundary layer, leading to increased mixing between the boundary layer and the core flow of hydrocarbon+steam mixture. It is conventional to lessen the pressure-drop of the hydrocarbon+steam mixture traversing radiant tubes which contain one or more mixing elements. For example, U.S. Pat. No. 7,799,963 describes a structure which provides a decreased pressure drop as a result of discontinuities in the mixing elements. Both the tube and the discontinuous mixing elements are formed from conventional steam cracker alloys such as 25 Cr/20 Ni, 25 Cr/35 Ni, 35 Cr/45Ni, or Incolloy™.
Nevertheless, there remains a need for heat transfer tubes that suppress the formation of chromium-carbide phases while providing improved heat transfer through the incorporation of mixing elements.